Ivo Kusák

Tracing of Concrete Hydration by Means of Impedance Spectroscopy (New Tool for Building Elements Testing)

Concrete setting and hardening designate the concrete grade. The impedance spectroscopy method, as one of the non-destructive testing method group, was used to characterize concrete specimens and track the changes in the concrete spectrum. Variances in the tan δ (f) and Im Z(f) or Re Z(f) of the specimens under investigation have been observed. The specimen quality has been described by means of the loss type prevailing in the material. The results of this study are expected to provide information about the correlation between the n-factor and the concrete setting time.

Comparison of Impedance Spectra of Concrete Recorded with Utilizing Carbon Transition Paste

NDT method of impedance spectroscopy was used to characterize concrete slab specimens without and with contact graphite compound coating. Differences in tanδ(f), ImZ(f) and ReZ(f) spectra for dry specimens were observed. Furthermore, based on the predominant loss type, the material quality was characterized. Our measurements confirmed the reproducibility of this method. Furthermore, the influence of the concrete specimen composition on their electric parameters was assessed.

Dielectric Properties of Concrete Specimens after Heat Stress

Concrete quality depends on various stress factors. The impedance spectroscopy method, as one of the non-destructive testing method group, was used to characterize concrete specimens and track the changes in the concrete spectrum after heat stress. The temperature range was between 0°C and 1200°C. Variances in the tan δ (f) and Im Z(f) or Re Z(f) of the specimens under investigation have been observed. The specimen quality has been described by means of the loss type prevailing in the material. The results of this study are expected to provide information about the correlation between the relative permittivity and changes in the structure for heat stress samples.

LOW-FREQUENCY NOISE MEASUREMENTS USED FOR QUALITY ASSESSMENT OF GaSb BASED LASER DIODES PREPARED BY MOLECULAR BEAM EPITAXY

Abstract The paper reports on a non-destructive method of reliability prediction for semiconductor lasers diodes GaSb based VCSE (vertical cavity surface emitting). Transport and noise characteristic of forward biased were measured in order to evaluate the new MBE (molecular beam epitaxy) technology. The results demonstrate that the lasers prepared by new MBE technology have higher quality than the samples prepared by using the classic MBE technology.

Impact-Echo Methods to Assessment Corrosion of Reinforced Concrete Structures

The aim of this paper is to evaluate the corrosion of reinforced concrete structure. To assess the state of corrosion we used frequency signal analysis, where we observed the changes in dominant frequencies and growth of damping factor λ. Tension pulse was produced in axis of the rod at one end and detected in axis at opposite end of rod. Diameter of steel rod was 8 mm. The article presents the results of measurements obtained during 24 months controlled degradation in aqueous NaCl solution.

Exposure of Mortars Modified with Rubber Aggregates and Polymer Admixtures to Acid Environments and Elevated Temperature Conditions

AbstractThe aim of the present paper was to study the effect of incorporating rubber aggregates and acrylic or EVA polymers on the properties of cement mortar exposed to acid (HCl, H2SO4, HNO3) and elevated temperature conditions. Compressive and bending strength tests, water absorption test, mass measurements, and visual inspection were performed. Impact-echo and impedance spectroscopy methods were used to investigate the effect of heat exposure. The use of rubber aggregates led to reduction of strength values and water absorption. The use of polymeric admixtures further decreased water absorption and inhibited compressive strength loss and delayed mass loss when the specimens were stored in acid solutions. The acoustic and electric measurements showed that the use of rubber aggregates or both rubber aggregates and polymers led to reduction of absorption frequency or relative permittivity, respectively. The drop of frequencies was attributed to the formation of microcracks, as well as to decomposition an...

Differences in Electrical Properties of Portland Cement and Alkali-Activated Slag Mortars

Alkali-activated slag is known as a building material for more than sixty years and is considered an alternative to Portland cement based binders. Compared to Portland cement it exhibits some superior properties such as higher resistance against chemical attack and exposure to elevated temperatures. Aluminosilicate binders are generally electrical insulators; however, electrical properties of building materials gain the importance in the new field of applications such as self-sensing or self-heating materials. This paper brings a comparison of the electrical properties, especially resistance and capacitance, between Portland cement and alkali-activated slag mortars. The measurements revealed that alkali-activated slag shows enhanced conducting properties due to the presence of mobile hydrated sodium ions and metallic iron microparticles.

Electrical Properties of Steel Fibre Reinforced Alkali-Activated Slag Composite

Alkali-activated slag is an alternative binder to the ordinary Portland cement. In order to improve its tensile properties steel fibres as dispersed reinforcement can be used. Since steel is very good conductor it changes the electrical properties of alkali-activated slag composite that can have a potential to be used as self-sensing material then. In this study up to 20% of steel fibres by mass of the slag was added to alkali-activated slag mortar and the mechanical properties, electrical resistance, capacitance and microstructure of the composites were investigated. The results showed that the best improvement of both the mechanical and electrical properties can be observed for the composite with 15% of steel fibres.

Electrical Properties of Alkali-Activated Slag Mortar with Carbon Fibres

Building materials with enhanced electrical properties gain the importance in the new field of applications such as self-sensing or self-heating materials. In this paper, 3 mm long carbon fibres were used as a conductive admixture to alkali-activated slag mortar in order to reduce its resistivity. The amount of carbon fibres was ranging from 0.5 to 4.0% of the slag mass and the effect of the conductive admixture on the mechanical properties, electrical impedance, specific conductivity, and microstructure of alkali-activated slag composite was investigated. Only 0.5% of carbon fibres caused a significant decrease in impedance of alkali-activated slag composite and the addition of 4% reduced the impedance by one order of magnitude for low AC frequencies. However, due to problematic dispersion and higher demand of mixing water, the mechanical properties were deteriorated, especially at higher content of carbon fibres.

Electrical properties of alkali-activated slag composite with combined graphite/CNT filler

Alkali-activated industrial by-products such as blast furnace slag are known to possess properties which are comparable to or even better than those observed for ordinary Portland cement. The combination of alkali-activated slag matrix with conductive filler introduces new functionalities which are commonly known for self-sensing or self-heating concrete. The present paper discusses the effect of the mixture of two different conductive fillers, graphite powder and carbon nanotubes (CNTs), on the electrical properties of alkali-activated slag mortars. Prepared samples were also tested for their mechanical properties and microstructure was investigated by means of mercury intrusion porosimetry and scanning electron microscopy. The percolation threshold for the resistance was reached for the mixture containing 0.1% CNTs and 8% graphite powder.